WASHINGTON -- Barack Obama and John McCain get their last chance Wednesday night to debate before tens of millions of voters about how they differ over remedies for the nation's most vexing issues: the economy, healthcare and the future of Medicare and Social Security.

But few experts expect much new insight from their 90-minute debate at 9 p.m. Eastern time.

Not only does the format discourage talk about details, but also on the stump, on their websites and in their ads, the campaigns have shown little desire to get too precise.

''I've been very disappointed in both of them for the lack of vision,'' said William Shughart, professor of economics at the University of Mississippi.

LOW EXPECTATIONS

He and others don't expect much depth.

''The debate format doesn't lend itself to detail or complexity,'' said Julian Zelizer, professor of history and public affairs at Princeton University.

Hofstra University in Hempstead, N.Y., is the site of the third and final debate. McCain and Obama will answer questions from moderator Bob Schieffer of CBS.

Schieffer says he'll press for specifics, but historians say he's got a tough job, because candidates tend to be vague, even in tough times.

''Franklin Roosevelt in the 1932 campaign was famously known for promising bold, persistent experimentation,'' without being too specific, said John Geer, editor of the Journal of Politics.

The recent model for substantive campaigning was probably 1992, when candidates spent months debating the economy and offered books laying out their proposed programs.

Since then the Internet and 24-hour news cycles have become pervasive, and ''candidates respond to the media,'' said David Carney, a Republican consultant who was White House political director under George H. W. Bush.

''In 1992 everyone had a book,'' he said. ``This year everyone has a blog.''

Medicare is projected for insolvency by 2019 and Social Security is seen as beginning to spend more than it collects beginning in 2017, but Obama and McCain offer no comprehensive plans.

Maya MacGuineas, president of the Committee for a Responsible Federal Budget, said campaigns ''are about making the country feel comfortable,'' and talking about curbing Medicare and Social Security costs is hardly feel-good politics.

The problem extends to how to best manage the nation's healthcare system.

McCain talks about his tax credit -- $2,500 for individuals and $5,000 for families -- but experts find the specifics sketchy at best.

The money would go to the insurer, but unused parts could go into an individual's health savings account. It also would require consumers to pay tax on the value of an employer's health benefits.

That's a tax increase, Obama charges.

Leonard Burman, director of Washington's Tax Policy Center, a nonpartisan research group, has found that eligible households would save an average of $1,241 next year. That kind of detail has not been not been widely reported or discussed during debates -- though it could have been.

PLAYING IT SAFE

Geer thought a seminal moment in the debates came last week when audience member Lindsey Trella, a breast-cancer survivor who lost her job and saw her insurance premiums jump, asked Obama if healthcare ``should be treated as a commodity.''

Obama gave a lengthy answer, saying that she and others could buy the same kind of insurance as federal employees -- without explaining what that means -- then criticized McCain's ideas. McCain promoted his tax-credit plan, saying it would make it easier for people to shop for more affordable coverage.

''Both candidates went into their standard stump speeches,'' Geer said. ``The woman wanted a sense of where they stood on health care. Bill Clinton would have been hugging her and making her understand his program, but both Obama and McCain missed their chance.''

Chances are, Zelizer said, they probably will again on Wednesday night.

''The candidates know the whole point of these debates,'' he said, ``is to say one or two things that can be replayed the next day.''

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